Operational amplifier having light sources and optically coupled photoconductor elements in output circuit thereof

ABSTRACT

A high output voltage operational amplifier having light sources connected to the output of the associated preamplifier stage. A plurality of the light sources are connected to be selectively responsive to the polarity of the operational preamplifier output. Individual photoconductor elements are optically coupled with each of the light sources and their respective values of resistance are dependent upon the amount of illumination thereof by the associated light sources. The photoconductor elements are electrically isolated from their respective light sources but are optically responsive thereto to provide an output signal that is dependent upon the amplitude and polarity of the output of the operational preamplifier.

United States Patent Inventor Appl. No. Filed Patented OPERATIONAL AMPLIFIER HAVING LIGHT SOURCES AND OPTICALLY COUPLED PHOTOCONDUCTOR ELEMENTS IN OUTPUT CIRCUIT THEREOF 16 Claims, 2 Drawing Figs.

[52] U.S. Cl. 250/210,

v 330/59 [51] lnLCI HOlj 39/12 [50] Field of Search 250/208. 209, 210; 330/59; 307/299, 117

[56] References Cited UNITED STATES PATENTS 3,283,157 11/1966 Blackmer 330/59X 3,248,549 4/l966 Sanabria 250/210 3,283,237 11/1966 Williamset a1. 250/209 3333,105 7/1967 Kossakowski et al 250/210 Primary Examiner-Walter Stolwein Attorney-Stockman, Sears and Santorelli ABSTRACT: A high output voltage operational amplifier having light sources connected to the output of the associated preamplifier stage. A plurality of the light sources are connected to be selectively responsive to the polarity of the operational preamplifier output. Individual photoconductor elements are optically coupled with each of the light sources and their respective values of resistance are dependent upon the amount of illumination thereof by the associated light sources. The photoconductor elements are electrically isolated from their respective light sources but are optically responsive thereto to provide an outputsignal that is dependent upon the amplitude and polarity of the output of the operational preamplifier.

OPERATIONAL AMPLIFIER HAVING LIGHT SOURCES AND OPTICALLY COUPLED PI-IOTOCONDUCTOR ELEMENTS IN OUTPUT CIRCUIT THEREOF BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates to a high output voltage operational amplifier having particular utility for uses that require lowpower, high output voltage performance.

2. Description of the Prior Art The prior art teaches the use of vacuum tubes or complex solid-state techniques in the production of operational amplifiers. These normally are costly and have high-power requirements.

SUMMARY OF THE INVENTION The operational amplifier according to the invention provides a low-cost, high output voltage operating amplifier. At least first and second light sources are connected in the output circuit of the operational preamplifier and are selectively energizable depending upon the polarity of the output therefrom. The light sources produce a degree of illumination depending upon the amplitude of the output of the operational preamplifier.

Individual photoconductor elements are optically coupled to each of the light sources and are responsive to the degree of illumination produced by the latter. A power supply and load circuit are operatively associated with the photoconductor elements to produce an output voltage across the load circuit that depends upon the polarity and amplitude of the operational preamplifier output. The light sources and their associated photoconductor elements are electrically isolated.

The photoconductor elements may be connected in a bridge-type arrangement wherein four such elements are employed in association with four associated light sources. A high'voltage supply and load resistance are operatively connected to the bridge arrangement to provide an operational amplifier output that is dependent upon the polarity and amplitude of the operational preamplifier output because the photoconductor elements are optically coupled to their respective light sources and are therefore responsive thereto.

The invention provides a relatively simple, low-cost circuit, having excellent common mode rejection and common mode voltage range. Further, it provides a high-gain operational amplifier having good output efficiency characteristics. With the use of commercially available components, an output voltage of :2000 volts DC, an output current of 1250 4. amperes, and an open loop gain of are realizable.

DETAILED DESCRIPTION OF THE INVENTION FIG. 1 shows a preferred embodiment of the invention. The amplifier stages comprising the operational preamplifier are designated P. The particular circuitry thereof is not shown because it is not essential for an understanding of the invention. However, the operational preamplifier has its output connected to the parallel connection of light sources L1 and L2, and light sources L3 and L4. Diodes D1 through D4 are respectively connected in series with light sources Ll through L4 to polarize current flow through the latter, depending upon the polarity of the output signals of the operational preamplifi- Assuming the output signals from the operational preamplifier are positive, diodes D1 and D3 will be biased to conduction and a current path for the output signals will be provided by the parallel-connected paths of the series connections of light source L1 and diode D1, and light source L3 and diode D3, to ground. Alternatively, if the output signals are negative, diodes D2 and D4 will be biased to conduction and the parallel paths comprising the series connections of light source L2 and diode D2, and light source L4 and diode D4, will provide a current path to ground for the negative output signals.

Therefore, light sources L1 and L3 are energized only in response to positive output signals from the operational preamplifier, and light sources L2 and L4 are energized only in response to negative output signals therefrom. The two sets of parallel-connected sources are thus selectively energizable depending upon the polarity of the output signal from the operational preamplifier.

The power supply for the operational preamplifier is designated by battery B, having its potential midpoint connected to the series connection of diodes D1 and D2; that is, to the cathode of diode D1 and the anode of diode D2. A group of photoconductor elements having values of resistance that vary inversely with the amount of light to which they are exposed are operatively associated with the individual light sources. In particular, photoconductor elements R1 through R4, connected in different arms of a bridge type of arrangement, are respectively operatively associated with light sources Ll through L4 and are responsive to the amount of light emanating therefrom.

As discussed above, only light sources L1 and L3 are energized in response to a positive output signal, and only light sources L2 and L4 are energized in response to a negative output signal. Therefore, because the light sources are optically coupled to their associated photoconductor elements, the resistances of photoconductor elements R1 and R3 will decrease in response to energization of light sources L1 and L3, while the resistances of photoconductor elements R2 and R4 will remain very high since they are not optically coupled to light sources L1 and L3. Similarly, light sources L2 and L4 are respectively optically coupled to photoconductor elements R2 and R4. Therefore, the resistances of the latter will decrease when light sources L2 and L4 are energized in response to a negative output signal, while the resistances of photoconductor elements R1 and R3 will remain very high since they are not optically coupled to light sources L2 and L4.

The photoconductor elements R1 through R4 are chosen such that their dark values of resistance (that is, the value of resistance when not illuminated by the corresponding light source) are sufficiently high such that the corresponding dark current is negligible. Accordingly, if a positive output signal is produced by the final stage of the operational preamplifier, light sources Ll and L3 will be energized thereby illuminating the associated photoconductor elements R1 and R3, respectively. The resistance of the latter will thereby decrease, while the resistance of photoconductor elements R2 and R4 will remain very high. A conduction path will thereby be provided for high voltage supply E through photoconductor elements R1 and R3.

Tracing this path out with respect to FIG. I, the current will flow from the positive supply terminal of high voltage supply E, through limiting resistor R5, photoconductor element R1, load resistor RL, photoconductor element R3 limiting resistor R6, and return to the negative supply terminal of high voltage supply E. A positive output voltage will therefore be produced across load resistor RL.

Similarly, should negative output signals be produced by the last stage of the operational preamplifier, light sources L2 and L4 will be energized. Consequently, the illumination produced by the latter will cause phtotoconductor elements R2 and R4 respectively to assume their low values of resistance. Therefore the resistances of photoconductor elements R2 and R4. will be small compared to that of photoconductor elements R1 and R3 and a conduction path through photoconductor elements R2 and R4 will be provided. Tracing this path out, the current will flow from the positive supply terminal of highvoltage supply E through limiting resistor R5, photoconductor element R4, load resistor RL, photoconductor element R2, limiting resistor R6, to the negative supply terminal of high voltage supply E. A negative output signal will thereby be produced across load resistor RL.

FIG. 2 shows another embodiment of the invention wherein two light sources having corresponding photoconductor elements are provided. Light sources L5 and L6 are connected to the output of the operational preamplifier. Diodes D5 and D6 are respectively connected in series with light sources L5 and L6 between the output of the operational preamplifier and ground. As similarly described with regard to FIG. 1, light source L5 will be energized in response to positive output signals from the operational preamplifier and light source L6 will be energized in response to negative output signals from the final stage of the operational preamplifier.

Just as in the embodiment of the invention described in relation to FIG. I, photoconductor elements R7 and R8 are respectively optically coupled through light sources L5 and L6. The high voltage supply is divided between the circuits comprising photoconductor elements R7 and R8. Voltage supply E1 is operative, when a positive output voltage is produced by the operational preamplifier to energize light source L5 to thereby decrease the resistance of photoconductor element R7. This causes a positive output to be produced at the output of the operational amplifier across load resistor RL.

Similarly, when a negative output voltage is produced by the operational preamplifier, voltage supply E2 is operative to produce a negative output voltage from the operational amplifier across load resistor RL. This is because when a negative output voltage is produced by the operational preamplifier, light source L6 is energized and since it is optically coupled to photoconductor element R8, the resistance of the latter decreases and a negative voltage is thereby produced across load resistor RL.

Resistors R5 and R6 are current-limiting resistances. Similarly, with respect to FIG. 2, resistors R9 and R10 are also current-limiting resistances. Further, voltage supplies El and E2 are typically equal in potential value.

The relative positioning of the light sources and their as sociated photoconductor elements shown in FIG. 1 are schematic representations. In particular, the relationship between the light sources and their respective photoconductor elements is such that each light source is selectively positioned to only affect its associated photoconductor element; it is optically isolated from the other photoconductor elements. Such means are known to those versed in the art and are therefore not discussed in this application since they are not relevant to an understanding of the invention.

The advantages of the bridge configuration of FIG. 1 compared to the two photoconductor elements of FIG. 2 are:

l. Only one high voltage supply is required.

2. The output voltage capability is increased. The maximum possible voltage across a photoconductor element approaches the supply voltage whereas in the configuration of FIG. 2 the maximum voltage across a photoconductor element approaches double the voltage of a single supply (assuming equal supply voltages).

3. The output power capability is increased because the dissipation is split between two photoconductor elements connected in series.

In both the circuits shown in FIGS. 1 and 2, the load current through resistor RL has a magnitude that is established by feedback to provide the required null at the operational preamplifier input. The latter is a conventional lowpower/low-level operational amplifier having an adequate output capability to feed the light sources connected as its output load. It could be a vacuum tube amplifier or any other type but it typically would be a solid-state amplifier. Its bandwidth must be restricted by either internal or external lag networks to compensate for the limited speed of response of the light source/photoconductor arrangement. If not, its output could be readily saturated by noise thus causing the light sources to illuminate simultaneously or at a rate where all would simultaneously exhibit low resistance thus shorting their power supply and causing nonlinear operation.

The omission of bias sources in conjunction with the use of incandescent lamps and series-connected diodes permits a noise level of about 1 volt peak to peak or greater to exist at the operational preamplifier output without illuminating the lamps, thus ensuring good speed of response in the operational preamplifier which permits better speed of response in the overall system.

With reference to FIGS. 1 and 2, a ground connection is indicated at both the power supply common connection for the operational preamplifier and the photoconductor circuit common connection. The described common connection would normally be utilized with most conventional feedback configurations. However, in the event that a suitable feedback technique is employed, the ground connections can be disconnected as required. Then the common mode rejection and the common mode voltage range would be limited only by the isolation capability of the power supplies feeding the operational preamplifier and the photoconductors output circuitry.

As described, a dead zone of 1 volt or greater exists at the output of the preamplifier. That is, an output in excess of 0.5 volts is necessary before the lamps light. This dead zone insures excellent efficiency in the photoconductor output circuit but can adversely influence-optimized transient performance, particularly in the event neon lamps are employed as the light sources. This dead zone can be reduced to any degree desired by placing in series with the related series connected diode, a DC source which forward biases the light source to the desired degree.

The light sources may comprise one of a plurality of elements capable of being driven by the operational amplifier. These include incandescent lamps, light-emitting diodes, gaseous discharge lamps, etc.

The incandescent lamp has the following advantages:

1. Low cost.

2. Good spectral match to the associated photoconductor element.

3. High gain (large change in output light with input volt;

age).

It has the disadvantages of:

1. Slow speed of response.

2. Poor efficiency.

However the first listed disadvantage 1 can be largely circumvented by the use of available lamps ofvery small physical size which exhibit very fast speed of response. The lightemitting diode has a narrow spectrum which constitutes a problem in matching its output to the spectral response of the photoconductor element. The neon lamp is well suited to use with available photoconductor elements but must be prebiased to avoid excessive crossover distortion.

Photoconductor elements or cells basically comprise linear resistors having a value of resistance that is an inverse function of incident illumination. Their spectral response is dependent upon the photoconductor and window employed. They typically display a peaked spectral response and care must be taken to match their response to their light source in this application. Their speed of response to an increase in incident light is significantly faster than their speed of response to a decrease in light.

The use of two light sources and two cells or four of each in the bridge configuration causes the system response to be related to the speed of response of the cell whose illumination is increasing, thus circumventing the limitation of the much slower increase in resistance associated with the cell whose illumination is decreasing. The two-lamp system also permits excellent electrical efficiency as normally only one lamp is lit and no significant power is dissipated by the dark cell.

Photoconductors exhibit instability as a function of temperature and past history. They recover their ultimate dark resistance quite slowly after illumination is removed. The above characteristics would tend to limit their utility as an amplifying device as they have poor stability. The effect of this instability is reduced to a negligible value, however, by preceding the lamp/cell circuit by an operational preamplifier of sufficiently high gain. The readily available amplifiers meet this requirement.

In the event the cell output (operational amplifier output) is shorted, a cell illumination can exist which could cause destructive dissipation in the photoconductor if full supply voltage were maintained across that photoconductor. The supply output capability must thereby be limited to prevent this destruction. This limiter can take the form of a series resistor.

Although best performance will be realized if the cells are energized by a supply which is well regulated against line fluctuations, the amplifier will automatically regulate against changes in static supply output. The amplifiers response to sudden supply changes will be limited by the finite speed of response of the cells. Satisfactory performance withan unregulated supply can generally be realized by employing an RC filter in the unregulated supply having a time constant sufficiently long that the cell can compensate for the slow change thus producing adequate overall stability.

The output capability of the indicated circuit is limited by the voltage rating and the power dissipation rating of available cells. The voltage-delivering capability may be increased by series connecting cells but these cells should be shunted by voltage-equalizing resistors whose resistance is low compared to the cell dark resistance because dark resistances are, in general, not well matched. Power dissipation can be increased by paralleling cells but care should be taken to match the cells and allowances should be made for possible mismatches because this type of cell does not lend itself to precise stable matching.

Certain variations in the arrangement of the light source means are possible. For example, with regard to FIG. 1, light sources L1 and L3 were stated to be respectively optically coupled to photoconductor elements R1 and R3, and light sources L2 and L4 were stated to be respectively coupled to photoconductor elements R2 and R4. It is possible that the parallel connection of light sources L1 and L3, however, act jointly by being optically coupled to both photoconductor elements R1 and R3. Similarly, parallel-connected light sources L2 and L4 may act jointly by being optically coupled to both photoconductor elements R2 and R4. Further, if desired, and depending upon the particular light sources employed, a single light source may be employed to act on each pair of associated photoconductor elements. Thus, with regard to FIG. 2, light source L5 may be optically coupled to both photoconductor elements R1 and R3 of FIG. 1, and light source L6 may be similarly coupled to both photoconductor elements R2 and R4.

Also with respect to FIG. 1, light sources L1 and L3 can be connected in series to the output of the operational 'preampli fier P. Light sources L2 and L4-can be similarly connected in series. Further, a single diode may be substituted for diodes D1 and D3. Similarly a single diode may be substituted for diodes D2 and D4. Other variations may be employed without departing from the scope of the invention.

lclaim:

1. An operational amplifier circuit arrangement comprising:

an operational preamplifier, first and second light source means connected to the output circuit of the operational preamplifier, the first light source means being selectively energizable in response to an operational preamplifier output of one polarity, the second light source means being selectively energizable in response to an operational preamplifier output of the opposite polarity, first, second, third and fourth photoconductor means connected to comprise the four arms of a bridge having values of resistance that vary with the amount of incident light applied thereto, the first light source means being optically coupled to the first and third photoconductor elements, the second light source means being optically coupled to the second and fourth photoconductor means, comprising opposite arms of the bridge,

load means connected between a first set of diagonals of the bridge,

power supply means connected between the second set of diagonals of the bridge, the load means providing an operational amplifier output of one polarity in response to energization of the first light source means and of the opposite polarity in response to energization of the second light source means.

2. An operational amplifier circuit as recited in claim 1 further comprising:

first and second rectifier means respectively connected in series with the first and second light source means to polarize the latter to be responsive to the selected polarity of operational preamplifier output.

3. An operational amplifier as recited in claim 2 wherein each of the photoconductor means have values of resistance that vary inversely with the amount of incident light applied thereto by the associated photoconductor means.

4. Anoperational amplifier as recited in claim 3 further comprising;

limiting means connected to limit the operational amplifier output.

5. An operational amplifier circuit arrangement as recited in claim 1 wherein the first light source means comprises the parallel connection of individual first and second light sources jointly optically coupled to both the first and third photoconductor means and the second light source means comprises the parallel connection of individual third and fourth light sources jointly optically coupled to both the second and fourth photoconductor means.

6. An operational amplifier as recited in claim 5 further comprising:

first, second, third and fourth rectifier means respectively connected in series with the first, second, third and fourth light sources to polarize the latter to be responsive to the selected polarity of operational preamplifier output.

7. An operational amplifier as recited in claim 6 wherein each of the photoconductor means have values of resistance that vary inversely with the amount of incident light applied thereto by the associated light source.

8. An operational amplifier as recited in claim 7 further comprising:

limiting means connected to limit the operational amplifier output.

9. An operational amplifier circuit as recited in claim 1 wherein the first light source means comprises the parallel connection of first and second individual light sources respectively optically coupled to the first and third photoconductor means, and the second light source means comprises the parallel connection of third and fourth individual light sources respectively optically coupled to the second and fourth photoconductor means.

10. An operational amplifier as recited in claim 9 further comprising: 7

first, second, third and fourth rectifier means respectively connected in series with the first, second, third and fourth light sources to polarize the latter to be responsive to the selected polarity of operational preamplifier output.

11. An operational amplifier as recited in claim 10 wherein each of the photoconductor means have values of resistance that vary inversely with the amount of incident light applied thereto by the associated photoconductor means.

12. An operational amplifier as recited in claim 11 further comprising:

limiting means connected to limit the operational amplifier output.

13. An operational amplifier circuit arrangement comprismg:

an operational preamplifier,

first and second light sources connected to the output of the operational preamplifier, the first light source being selectively energizable in response to an operational preamplifier output of one polarity, the second light source being selectively energizable in response to an operational preamplifier of the opposite polarity,

series connected first and second photoconductor means optically coupled to the first and second light sources respectively, and having values of resistance that vary with the amount of incident light applied thereto by the first and second rectifieFrrEans respectively connected in latter, series with the first and second light sources to polarize a load circuit including a'common load resistance conthe latter to be responsive to the selected polarity of nected to the common connection of the series-conoperational preamplifier output.

nected first and second photoconductor means, a power 15. An operational amplifier circuit arrangement as recited supply means nn t d t h fi d d in claim 14 wherein the first and second photoconductor photoconductor m ans and th l d i i to id an means have values of resistance that vary inversely with the operational amplifier output of one polarity in response amount Ofmcldef light PP P- to energization of the first light source and of the opposi 16. An operational amplifier as recited in claim 15 further polarity in response to energization of the second light 10 Pf "8 Source means, limiting means connected to limlt the operational amplifier 14. An operational amplifier as recited in claim 13 further Outputcomprising: 

1. An operational amplifier circuit arrangement comprising: an operational preamplifier, first and second light source means connected to the output circuit of the operational preamplifier, the first light source means being selectively energizable in response to an operational preamplifier output of one polarity, the second light source means being selectively energizable in response to an operational preamplifier output of the opposite polarity, first, second, third and fourth photoconductor means connected to comprise the four arms of a bridge having values of resistance that vary with the amount of incident light applied thereto, the first light source means being optically coupled to the first and third photoconductor elements, the second light source means being optically coupled to the second and fourth photoconductor means, comprising opposite arms of the bridge, load means connected between a first set of diagonals of the bridge, power supply means connected between the second set of diagonals of the bridge, the load means providing an operational amplifier output of one polarity in response to energization of the first light source means and of the opposite polarity in response to energization of the second light source means.
 2. An operational amplifier circuit as recited in claim 1 further comprising: first and second rectifier means respectively connected in series with the first and second light source means to polarize the latter to be responsive to the selected polarity of operational preamplifier output.
 3. An operational amplifier as recited in claim 2 wherein each of the photoconductor means have values of resistance that vary inversely with the amount of incident light applied thereto by the associated photoconductor means.
 4. An operational amplifier as recited in claim 3 further comprising; limiting means connected to limit the operational amplifier output.
 5. An operational amplifier circuit arrangement as recited in claim 1 wherein the first light source means comprises the parallel connection of individual first and second light sources jointly optically coupled to both the first and third photoconductor means and the second light source means comprises the parallel connection of individual third and fourth light sources jointly optically coupled to both the second and fourth photoconductor means.
 6. An operational amplifier as recited in claim 5 further comprising: first, second, third and fourth rectifier means respectively connected in series with the first, second, third and fourth light sources to polarize the latter to be responsive to the selected polarity of operational preamplifier output.
 7. An operational amplifier as recited in claim 6 wherein each of the photoconductor means have values of resistance that vary inversely with the amount of incident light applied thereto by the associated light source.
 8. An operational amplifier as recited in claim 7 further comprising: limiting means connected to limit the operational amplifier output.
 9. An operational amplifier circuit as recited in claim 1 wherein the first light source means comprises the parallel connection of first and second individual light sources respectively optically coupled to the first and third photoconductor means, and the second light source means compriseS the parallel connection of third and fourth individual light sources respectively optically coupled to the second and fourth photoconductor means.
 10. An operational amplifier as recited in claim 9 further comprising: first, second, third and fourth rectifier means respectively connected in series with the first, second, third and fourth light sources to polarize the latter to be responsive to the selected polarity of operational preamplifier output.
 11. An operational amplifier as recited in claim 10 wherein each of the photoconductor means have values of resistance that vary inversely with the amount of incident light applied thereto by the associated photoconductor means.
 12. An operational amplifier as recited in claim 11 further comprising: limiting means connected to limit the operational amplifier output.
 13. An operational amplifier circuit arrangement comprising: an operational preamplifier, first and second light sources connected to the output of the operational preamplifier, the first light source being selectively energizable in response to an operational preamplifier output of one polarity, the second light source being selectively energizable in response to an operational preamplifier of the opposite polarity, series connected first and second photoconductor means optically coupled to the first and second light sources respectively, and having values of resistance that vary with the amount of incident light applied thereto by the latter, a load circuit including a common load resistance connected to the common connection of the series-connected first and second photoconductor means, a power supply means connected to the first and second photoconductor means and the load circuit to provide an operational amplifier output of one polarity in response to energization of the first light source and of the opposite polarity in response to energization of the second light source means,
 14. An operational amplifier as recited in claim 13 further comprising: first and second rectifier means respectively connected in series with the first and second light sources to polarize the latter to be responsive to the selected polarity of operational preamplifier output.
 15. An operational amplifier circuit arrangement as recited in claim 14 wherein the first and second photoconductor means have values of resistance that vary inversely with the amount of incident light applied thereto.
 16. An operational amplifier as recited in claim 15 further comprising: limiting means connected to limit the operational amplifier output. 